In the prior art, there is known an image generation system which can generate an image viewable from a given viewpoint in an object space that is a virtual three-dimensional space Such an image generation system is very popular as a system which can implement a so-called virtual reality. If the image generation system is for a racing game, a player can enjoy a three-dimensional game by controlling and running a racing car (or object) in the object space to compete with the other racing cars controlled by the other players or computer.
In such an image generation system, it is an important technical subject that a more realistic image is generated to improve the virtual reality for the players.
For example, the brightness of an object surface is calculated based on the reflection of light from a light source on the object surface, and a light source processing is made to set the color according to the brightness. Thus, the object may be shaded to provide a more realistic three-dimensional image. However, this increases the processing load in the image generation.
On the other hand, the image generation system of the prior art was subjected to various improvements for reducing the processing time in generation of the image, since view images must be generated in real time within the virtual three-dimensional space in accordance with various changing positions of a moving object.
For example, an image representing a tree located in the background of a scene may be generated by mapping the texture of the tree onto a simple object consisting of one plate-like polygon, rather than formation of a three-dimensional object constructed by a number of polygons. In such a manner, the number of polygons to be processed can be reduced, resulting in great reduction of the processing time.
However, such a simple object was generated without being subjected to any light source processing leading to increase of the processing time since it was used to reduce the processing time. Therefore, the tree or the like constructed by the simple object would always be displayed with the same tone and brightness irrespectively of the positional relationship between the tree and the light source. Inherently, the brightness on the object surface depends on the reflection of light which was variable according to the angle between the direction of the light and each of the polygon surfaces constructing the object. Since the simple object is always displayed with the same brightness irrespectively of the incident angle of the light, an unnatural image will be generated with less reality.